Apparatus which use ultrasonic energy for the diagnostic imaging of body and other structures are well known. Typically, pulses of ultrasound energy are directed into and reflected or scattered back from body tissues. The amplitude or the envelope of the reflected signals or echoes is detected and displayed to form images conforming to the outlines of various body organs and tumors, for example, when they are present.
Unfortunately, the images are not as clear as would be desirable, are often lacking in fine detail, and are obscured by large and bright laterally extending streaks or image distortions.
In U.S. Pat. No. 4,543,826, granted Oct. 1, 1985, is disclosed a system in which the reflected signals are frequency demodulated and applied to the scan converter to form the image. It is understood that the detail provided by such a system is improved, and that the laterally extending streaks mentioned above are eliminated, but the contrast and macroscopic image effect is poor. Also disclosed in U.S. Pat. No. 4,543,826 are systems in which the frequency demodulated signals are combined with the amplitude demodulated signals by simple summation, and a color display system wherein one signal is employed to control the "luminance" or brightness and the other is used to control the "chrominance" or color.
However, I have determined that, during certain portions of the pulse echo cycle, when the amplitude of the reflected signal is small, the frequency value takes wild excursions, resulting in a very noisy signal with very little dynamic range.
Accordingly, a principal object of the present invention is to provide an ultrasonic imaging system utilizing both amplitude and frequency demodulated signals, which avoids and overcomes the disadvantages of the prior art systems of the types mentioned above.